1. Field of the Invention
Ophthalmic lens casting with particular reference to method and apparatus for making casting mold pieces.
2. Description of Prior Art
Methods of producing the traditional glass casting molds used in the manufacture of ledged multifocal lenses involve, in each case, either machining the mold from one piece of glass or fusing one or more sections of glass together or onto a preformed casting surface of a major mold component. U.S. Pat. Nos. 3,109,696; 3,460,928; and 3,555,126 are exemplary. These methods have serious drawbacks, i.e. in addition to the tediousness and high cost of fabrication, ledged glass molds suffer from fracture limitations and other mechanical shortcomings not expected of plastic molds.
Plastic casting molds produced from a single master die offer the advantage of accurate configurational duplication and economical replacement as needed.
Heretofore, however, master dies required for producing plastic multifocal casting molds were costly, tedious and difficult to manufacture. The problems and limitations experienced in making glass casting molds were similarly encountered. For example, considerable difficulty was found in producing acceptable multifocal ledge cornering and finish. Moreover, an entire line of dies offering a reasonable choice of lens distance viewing and reading portion power combinations has required an excessive number of master dies, e.g. 72 master dies were needed for supplying six distance viewing surface curvatures with twelve reading portion combinations, trifocal combinations excluded. Also, traditional mold or die making practices do not permit bringing multifocal ledge height to zero at the mold center.
In view of the above, this invention has an objective of facilitating the manufacture of cast ophthalmic lenses, more particularly by improvement in master molding dies from which molded plastic multifocal lens casting pieces may be made.
Another object is to provide a novel master die construction and method of making the same wherewith the present tediousness and costliness of die manufacture and inventory are overcome.
Another object is to provide means and method for making molded multifocal casting pieces and lenses cast therefrom of designs heretofore considered impractical, if not impossible, to produce, e.g. straight-line one-piece multifocals with zero ledge thickness at center may be made according to the invention.
Other objects and advantages of the invention will become more readily apparent from the following description.